This invention relates to the measurement of finish quality of a painted surface and particularly to a method and apparatus of measuring paint surface irregularities such as orange peel and texture.
Painted articles have varying degrees of surface roughness or irregularity which may be unacceptable to users of the articles. Automotive vehicles and appliances, for example, have high quality requirements for paint appearance. A common type of paint condition is orange peel, which comprises roughly sinusoidal surface undulations. To determine acceptable quality of a painted product it is necessary to measure the severity of orange peel and compare it to known standards. Orange peel is a paint condition in which the paint surface includes numerous bumps having wavelengths in the range of approximately 0.5 to 10 millimeters. Orange peel gives the painted surface a rough, hilly appearance, in which the individual bumps creating the roughness can be seen. The common practice for measuring orange peel is to subjectively compare standard panels having varying degrees of orange peel with the article being inspected. Such subjective inspection is marked by inconsistency due in part to different inspectors.
Texture is associated with bumps and linear striations in the paint surface having wavelengths smaller than 0.5 millimeters. The effect of texture is seen most easily in its absence, in that a painted surface lacking texture has a shiny "wet look". Where texture is present, it is difficult to quantify using the visual methods commonly used for orange peel, since the individual bumps and striations which create the texture are too small to be individually seen without the assistance of special instruments. Essentially, a painted surface without the "wet look" has texture; but there is no prior art method for quantifying texture in manufacturing inspections.
There have been some instruments proposed to measure orange peel. For example, the paper "New Portable Orange Peel Meter for Paint Coatings", Matsuta et al, ISCC Conf., 1988, discusses a system which projects a rectangular wave pattern onto a surface, detects the reflected pattern by a linear image sensor and analyzes the distortion in the image. The meter was applied to surfaces with a radius of curvature of 1 meter or more. The measured distortion may be the result of dust or other surface flaws in addition to orange peel, and is limited to relatively large features.
An additional prior art publication, U.S. Pat. No. 4,629,319 to Clarke et al discloses apparatus for detecting surface defects such as dents, creases, low spots, and flat spots, as well as, allegedly, paint defects such as orange peel. An embodiment of the apparatus described in the Clarke et al disclosure scans a light beam across a painted surface and receives the beam reflected from the painted surface on an area detector which generates a signal indicative of the position on the detector surface struck by the reflected beam. The beam striking the area detector, over time, generates a signal which varies to indicate surface irregularities as the beam is scanned. One component of this signal, which is considered "background noise" in the detection of large scale dents and bumps in the surface underlying the paint, may be considered indicative of the paint surface itself and may thus indicate the degree of orange peel.
It is, however, necessary to converge the reflected light beam on the area detector as the beam is scanned, and especially if the surface is curved or otherwise changes its orientation to the light beam generating apparatus. The Clarke et al apparatus uses a retroreflective screen to redirect the reflected beam back along its path to be re-reflected from the painted surface back to a beam splitter and, from there, to the area detector. In this way, a portion of the beam is always redirected back toward the source and beam splitter (and thus the area detector) regardless of the orientation or curvature of the painted surface to the apparatus or the scan direction.
However, as useful as this apparatus may be for detection of large scale surface effects such as bumps, dents, flat spots, etc. in the metal or plastic panel underlying the paint, it becomes progressively less useful as the size of the surface irregularities to be detected becomes smaller. Although the patent disclosure states that it may be used to detect orange peel, it further states that a resolution of 0.05 inches (1.25 mm) is sufficient for most geometric defects of interest. This would be sufficient for the larger sizes of orange peel defects (1.25 to 10 mm) but not for the smaller sizes (0.5 to 1.25 mm) and certainly not for the even smaller irregularities of texture. Actually, its usefulness for any accurate orange peel measurement is questionable. The basic reason for this is the second reflection of the beam from the painted surface after being redirected back by the retroreflective screen. The beam after its first reflection from the painted surface may contain the information desired to determine the degree of orange peel and/or texture, provided that the beam is focused to a sufficiently small size at the surface to be significantly affected by the irregularities to be detected. However, the retroreflective screen allows the beam to spread in a cone of a few degrees, so that the beam hitting the painted surface the second time on its way back to the beam splitter has a much larger spot area on the surface. Different parts of the beam strike small parts of the surface having greatly different slopes and orientations and are scattered, so that the information in the beam from the first reflection is significantly degraded by the second reflection. The effect is a loss of detail which obscures irregularities of small size.
It is thus desired to direct the beam in such a way that it converges on the area detector, regardless of the changing scan direction and possibly changing orientation or curvature of the painted surface, after only a single reflection from the painted surface and, preferably, without the use of a retroreflective screen.